This invention relates to communication switching systems. More particularly, it is concerned with switching networks employing several stages of solid state crosspoint switching circuits.
Crosspoint switching arrays employing solid state devices have been developed for use in switching networks in communication systems. Switching arrays employing controlled latching semiconductor devices such as silicon controlled rectifiers (SCR's) are described and claimed in U.S. Pat. No. 3,456,084 entitled "Switching Network Employing Latching Type Semiconductors" which issued on July 15, 1969 to Ernest F. Haselton, Jr. Switching circuits which are particularly amenable to fabrication as monolithic integrated circuits are described and claimed in U.S. Pat. No. 3,826,873 entitled "Switching Circuit Employing Latching Type Semiconductor Devices and Associated Control Transistors" which issued on July 30, 1974 to A. Frederick Susi and U.S. Pat. No. 3,819,867 entitled "Matrix Employing Semiconductor Switching Circuit" which issued on June 25, 1974 to Jeremiah P. McCarthy and William Salmre.
The operating characteristics of the controlled latching semiconductor devices employed in these switching circuits are well-known. When a device is in the nonconducting condition and properly biased, current flow to the gate electrode causes current to flow between the anode and cathode turning the device on. The device continues to conduct after termination of the gate current as long as the current from the anode to the cathode exceeds a certain minimum current called the holding current. The device is restored to the nonconducting condition by momentary reduction of the current flowing therethrough to below the holding current.
As is well understood from the art of communication switching, solid state crosspoint switching circuits are arranged in networks so as to provide connections between signal transmission lines of different sets. Frequently the crosspoint switching circuits of a network are arranged in several stages so that a complete signal transmission signal path between two selected signal transmission lines require the activation of a crosspoint switching circuit in each of the several stages. Typically, biasing of the controlled latching semiconductor devices in a network is provided through the series connection of controlled latching semiconductor devices from stage to stage. That is, when a crosspoint switching circuit in each stage is selected to provide a signal transmission path through the network, biasing current is supplied to the controlled latching semiconductor devices of the selected switching circuits in series.
It has been found that when selected crosspoint switching circuits in several stages are turned on simultaneously, a problem of "current starving" may exist. Triggering current to the gate electrodes of the controlled latching semiconductor devices may deplete the series biasing current through the controlled latching semiconductor devices sufficiently that one or more of them fails to turn on. Sufficient current may be made available by employing sufficiently large biasing current sources. However, large biasing current sources introduce problems of high power consumption and dissipation together with related problems of power supply design and operation.